1. Field of the Invention
The present invention relates in general to a hydraulically operated brake system, and more particularly to such brake system provided with a high-pressure source adapted to generate a pressure higher than a pressure generated by an operator-controlled pressure generating device such as a master cylinder. For instance, the high-pressure source may be a servo pressure generator to be activated by a torque which is transmitted from a disc rotor to a brake pad of a braking device during operation of the braking device by the pressure generated by the operator-controlled pressure generating device, so that the higher servo pressure is applied to the braking device.
2. Discussion of the Prior Art
A hydraulically operated brake system provided with such a servo pressure generating device is known according to JP-A-57-30647, which includes (a) an operator-controlled pressure generating device for generating a fluid pressure depending upon an operation of a brake operating member, (b) a braking device which has a brake pad, a pad support member supporting the brake pad, and a brake cylinder operated by a pressurized fluid to force the brake pad against a disc rotor, and which is attached to a stationary member near the disc rotor, (c) a servo pressure generating device activated by a torque transmitted to the pad support member from the disc rotor during operation of the brake cylinder, to generate a servo pressure, and (d) a pressure control valve having a first pressure chamber connected to the operator-controlled pressure generating device, a second pressure chamber connected to the braking device and a third pressure chamber connected to the servo pressure generating device. The pressure control valve operates to effect selective connection and disconnection of the first, second and third pressure chambers, for controlling the pressure in the second pressure chamber such that the pressure in the second pressure chamber is higher than and relating to the pressure in the first pressure chamber.
In this type of hydraulically braking system, a torque is transmitted to the pad support member from the disc rotor when the brake pad is forced against the rotor, and the servo pressure generating device is activated by the torque transmitted to the pad support member, whereby the servo pressure is generated by the servo pressure generating device.
When the torque exceeds a given value, the servo pressure becomes higher than the pressure generated by the operator-controlled pressure generating device such as a master cylinder. The pressure control valve is adapted to effect selective connection and disconnection of the operator-controlled pressure generating device, braking device and servo pressure generating device, so that the pressure in the braking device is controlled such that the pressure in the braking device is higher than and relating to the pressure in the operator-controlled pressure generating device.
When the vehicle is stopped with the brake operating member kept operated, the torque transmitted from the disc rotor to the pad-support member is lowered, or a reverse torque in the opposite direction is transmitted to the pad support member. In this case, the servo pressure generated by the servo pressure generating device is lowered, and may be lower than the atmospheric pressure.
When the vehicle is braked and stopped, a force acts on the vehicle body in a reverse direction opposite to the running direction, immediately after the vehicle is brought to a stop. During deceleration of the vehicle, an advancing movement of the vehicle is restricted, while the vehicle body tends to continue an advancing movement due to an inertia. As a result, elastic members such as those used in a suspension system connecting the wheels and the vehicle body undergo elastic deformation. When the inertia of the vehicle body is zeroed upon stopping of the vehicle, the elastic members which have elastically deformed are restored to their original states, causing the vehicle body to be moved relative to the stopped wheels in the direction opposite to the direction in which the vehicle has run. Consequently, the vehicle body is moved backward, causing a torque to act on the wheels in the reverse direction. This reverse torque is transmitted to the pad support member through the disc rotor, whereby the pressure in the servo pressure generating device is lowered.
When the vehicle is stopped on an uphill road surface, too, a torque acts on the wheel in the reverse direction, and a reverse torque is transmitted to the pad support member of the braking device.
If the servo pressure generated by the servo pressure generating device is reduced or lowered below the atmospheric pressure, the pressure in the braking device may be higher than the servo pressure. In this case, the brake fluid is fed into the servo pressure generating device. In the meantime, the pressure in the operator-controlled pressure generating device (in the first pressure chamber in the pressure control valve) is maintained at a relatively high level because the brake operating member is kept in an operated position to hold brake application to the stopped wheels. Accordingly, the pressure in the braking device (in the second pressure chamber of the pressure control valve) is made lower than the pressure in the operator-controlled pressure generating device (in the first pressure chamber), and the brake fluid is discharged from the operator-controlled pressure generating device, whereby the brake operating member is moved toward its fully operated position unexpectedly to the vehicle operator. That is, the operating stroke of the brake operating member is abruptly increased unexpectedly to the vehicle operator. This gives the vehicle operator an anxiety about the reliability of the brake system.
To avoid such drawback, the pressure control valve has not only a primary function of controlling the pressure in the second pressure chamber depending upon the pressure in the first pressure chamber, but also a secondary function of restricting the amount of increase in the operating stroke of the brake operating member due to a discharge flow of the fluid from the operator-controlled pressure generating device. Described more specifically, two shut-off valves are disposed in series between the second and third pressure chambers. One of these shut-off valve functions to control the pressure in the second pressure chamber, while the other shut-off valve is adapted to be closed to prevent a discharge flow of the brake fluid from the braking device toward the servo pressure generating device when the pressure in the third pressure chamber is lowered below a predetermined level.
However, the latter shut-off valve is closed only after the pressure in the second pressure chamber is lowered below the pressure in the first pressure chamber (in the operator-controlled pressure generating device) as a result of a discharge flow of the brake fluid from the braking device, which occurs due to a pressure reduction in the servo pressure generating device (in the third pressure chamber) upon stopping of the vehicle with the brake operating member kept operated. This arrangement is not capable of completely preventing an abrupt increase of the operating stroke of the brake operating member, and is not still satisfactory to relieve the vehicle operator from the anxiety about the operating reliability of the brake system.
The conventional brake system described above suffers from another problem. That is, the pressure control valve is normally placed in a first state for connection of the second pressure chamber to the first pressure chamber and disconnection of the second pressure chamber from the third pressure chamber, for controlling the pressure in the second pressure chamber to be equal to the pressure in the first pressure chamber. The pressure control valve is operated from the first state to a second state when the pressure in the first pressure chamber exceeds a predetermined level. In the second state, the second pressure chamber is disconnected from the first pressure chamber, and connected and disconnected to and from the third pressure chamber, for controlling the pressure in the second pressure chamber to be higher than and relating to the pressure in the first pressure chamber. The problem is encountered when the pressure control valve is operated from the first state to the second state.
Described in detail, when the pressure control valve is operated to the second state, the brake cylinder in the braking device is disconnected from the operator-controlled pressure generating device (such as a master cylinder operated by a brake pedal), and connected and disconnected to and from the servo pressure generating device (provided as a high-pressure source) for controlling the pressure in the brake cylinder to be higher than the pressure in the operator-controlled pressure generating device. As a result, the pressure in the wheel cylinder is suddenly increased, and the controllability of the braking force is deteriorated.
The deterioration of the braking force controllability is serious particularly where the pressure in the operator-controlled pressure generating device at which the pressure control valve is operated from the first state to the second state is comparatively low. Namely, when the depression force acting on the brake operating member is comparatively small, the vehicle operator does not expect an abrupt change in the braking force produced by the braking device, and an intricate control of the braking force is generally required.
If the braking force is suddenly increased upon transition of the pressure control valve from the first state to the second state during braking of the vehicle on a road surface having a relatively low coefficient of friction, the vehicle wheels tend to slip to a considerable extent.